(Adapted from the applicant's abstract) This is a multicenter collaborative study to determine the sensitivity, specificity, positive and negative predictive value of contrast enhanced spiral computed tomography (spiral CT) for the diagnosis of acute pulmonary embolism (PE). Pulmonary embolism is common, yet frequently undiagnosed and fatal. Spiral CT is a nearly noninvasive test that offers the possibility of a definitive diagnosis of PE by showing the outline of the thrombus in a pulmonary artery. However, its utility in the diagnosis of PE is unknown. Even though spiral CT has not been adequately validated, physicians at many hospitals now use it as a definitive and only diagnostic test. This could lead to overtreatment or undertreatment, both of which have serious potential complications. The role of spiral CT in the diagnosis of PE requires an accurate evaluation which will be made in this investigation by a composite of diagnostic tests including pulmonary angiography, ventilation/perfusion lung scans, venous compression ultrasound, and outcome analysis. The strengths of this proposal are: 1) the importance of the problem of PE in terms of lives lost from underdiagnosis and major bleeding from overdiagnosis; 2) the potential applicability of a new technology for the diagnosis of PE that will have widespread availability; 3) the protocol parallels standard diagnostic strategy without subjecting the patients to risk entirely for the acquisition of data and 4) the team of investigators is strong and has collaborated previously in successful research in PE.